Differences in susceptibility to infection by S. mutans strains Ingbritt (serotype c) and OMZ-65 (serotype d/g) were observed in seven human subjects. Whole saliva, collected from the subjects, induced aggregation of Ingbritt but not OMZ-65 cells. Saliva from humans who resisted infection by S. mutans aggregated Ingbritt cells to a greater degree than did the saliva from humans who were readily infected. Saliva from subjects most resistant to infection supported the adsorption of the highest number of either Ingbritt or OMZ-65 cells to hydroxylapatite beads. In contrast, saliva from humans most heavily infected caused low numbers of Ingbritt and OMZ-65 cells to adsorb to the beads. The evidence suggests that the ability of the saliva to aggregate S. mutans may be related to infection by these bacteria.